this version is based on code by xiaolai, modified by Joy on Jun 1, 2013

version 1.1

write seems to work well, but there is something wrong with read.

version 1.2

now the read works!:)

but ther are still some problems nto be solved.
  1) the read data has to be copied to the user space;
  2) data structure is not perfect;
  3) memory management.